My friend and I just finished Into the Nest on Legend with our new weapons from Back to Ubersreik equipped, and whilst I got credit for the completion, my friend did not. I was playing Kruber and he was playing Bardin. His weapons were the dual hammers and dual axes.
Is there any known bug about credit not triggering for these maps? Does it matter if you’re the host or not? Is there anyway credit could still be awarded to him?
Did he have the dual hammers in the 2nd slot? That’s my only guess
I’m not sure, but I was starting to think that might be the case. I may ask him next time he’s on. He was just done for the night when that happened.
Unfortunately we’re not able to credit him due to limitations in our backend. 
Chances are his dual hammers were equipped in the 2nd slot as mentioned above, which is a known issue internally. My apologies for the inconvenience.
For all heroes, the first slot is the melee slot and the second slot is the ranged slot. In order to receive credit for any of the new weapons, the weapon must be equipped in the melee slot.
In practice, this really only affects Slayer Bardin, for whom equipping a weapon in the second or “ranged” slot means it is not subject to any progress towards Okri’s Challenges.